1. Field of the Invention
This invention relates generally to reciprocating piston type internal combustion (I.C.) engines for motor vehicles. More specifically it relates to I.C. engines having variable compression ratio connecting rods, especially to systems, mechanisms, and strategies for operating a connecting rod to different compression ratios while an engine is running.
2. Background Information
A gasoline engine whose compression ratio remains invariant as operating conditions change is said to be knock-limited. This means that the compression ratio built into the engine design must be selected to avoid objectionable engine knock that would otherwise occur during certain conditions of engine operation if the compression ratio were larger. However, those conditions that give rise to engine knocking in a motor vehicle typically prevail for only limited times as the vehicle is being driven. At other times, the engine could operate with better efficiency, and still without knocking, if the compression ratio could be made higher, but unfortunately the engine is incapable of achieving more efficient operation during those times because its compression ratio cannot change.
Certain technologies relating to reciprocating piston I.C. engines having variable compression ratio pistons and connecting rods are disclosed in various patents, including U.S. Pat. Nos. 1,875,180; 2,376,214; 4,510,895; 4,687,348; 4,979,427; 5,562,068; and 5,755,192. Various reasons for employing such technologies in I.C. engines have been advanced in those documents. One reason is to improve efficiency by enabling an engine that is relatively more lightly loaded to run at a compression ratio that is higher than a compression ratio at which the engine operates when running relatively more heavily loaded.
The compression ratio of an engine can be varied by varying the overall effective length of a connecting rod and piston. Change in overall effective length may be accomplished in either the connecting rod, or the piston, or in both. The foregoing patents describe various mechanisms for varying overall effective length.
U.S. Pat. No. 5,562,068 discloses a variable compression ratio connecting rod where adjustment of effective length takes place at the large end. Adjustment is performed via an eccentric ring that is generally coincident with a crank pin, but can be selectively locked to the crank pin and to the large end of the rod. When locked to the crank pin, the eccentric ring assumes a position that causes the rod to have a longer effective length and hence a higher compression ratio. When locked to the rod, the eccentric ring assumes a position that causes the rod to have a shorter effective length and hence a lower compression ratio.
The invention disclosed in one of the commonly owned patent applications that is incorporated herein by reference changes the connecting rod effective length at the large end of the connecting rod so that the incorporation of variable compression ratio by length change does not adversely contribute to the reciprocating mass of an engine in a way that might otherwise create unacceptable imbalance. That connecting rod comprises an assembly that contains a first part, a second part, and a third part assembled together to form the large end of the connecting rod assembly and provide a variable length for the connecting rod assembly. The first part is a semi-circular cap. One of the second and third parts is fastened tight to the first part. Guides disposed at opposite sides of the large end operatively relate the other of the second and third parts and the fastened parts to provide for relative sliding motion between the other of the second and third parts and the fastened parts over a limited adjustment range to change the length of the connecting rod assembly. Such a length change mechanism does not employ an eccentric ring as in U.S. Pat. No. 5,562,068.
The present invention relates to novel systems, mechanisms, and strategies: for operating a connecting rod, especially a connecting rod of the general type disclosed in the above referenced commonly owned patent application, to positions of different length while an engine is running, thereby changing the compression ratio; for locking the connecting rod in one position until it is desired to change length; for unlocking the connecting rod when a length change is desired; for utilizing inertial force to perform the length change; and for locking the connecting rod in another position upon completion of the length change.
The invention utilizes novel mechanical locking mechanisms to lock the connecting rod in its positions of different length. Operation of the locking mechanisms is accomplished by hydraulic pressure, using engine motor oil.
A connecting rod employs two such locking mechanisms. With both locking mechanisms unlocked, the centerline of the large end of a connecting rod is free to move between a position of concentricity relative to the centerline of a crank pin on which it is mounted via a bearing retainer and a position of eccentricity relative to the crank pin centerline.
When a connecting rod is in an extended position that endows it with a longer effective length, a higher compression ratio results. When a connecting rod is in a retracted position that endows it with a shorter effective length, a lower compression ratio results. When a connecting rod is in one of these two positions, one of the two locking mechanisms is locking the connecting rod to the bearing retainer while the other locking mechanism is unlocked.
To change length from an initial length, be that length extended length or retracted length, hydraulic pressure is applied to cause the one locked mechanism to unlock, freeing the connecting rod for re-positioning the centerline of its large end relative to the centerline of the crank pin on which it is mounted. with the connecting rod unlocked from the bearing retainer, inertial force acting on the connecting rod is effective to move it such that the centerline of the large end is re-positioned relative to the centerline of the crank pin, thereby changing the effective length of the connecting rod from the initial length to a new length. Upon completion of the length change, the hydraulic pressure that was effective to unlock the one locking mechanism now becomes effective to lock the other locking mechanism, thereby locking the connecting rod to the bearing retainer in the position of new length. The change in position of the large end centerline relative to the crank pin centerline prevents re-locking of the one locking mechanism that was initially unlocked to initiate the length change, and so that one locking mechanism remains unlocked while the connecting rod is in the new length position.
For changing the effective length from the new length back to the initial length, the application of hydraulic pressure is discontinued, causing the other locking mechanism to unlock and free the connecting rod for re-positioning the large end on the crank pin. With the connecting rod unlocked from the bearing retainer, inertial force acting on the connecting rod is effective to reposition it on the crank pin, thereby restoring the effective length to the initial length. Upon completion of the length change, spring force acting on the lock pin becomes effective to lock the one locking mechanism, thereby locking the connecting rod to the bearing retainer with the connecting rod in the initial length position. The length change prevents the other locking mechanism from being re-locked and so it remains unlocked while the connecting rod is in the initial length position.
One generic aspect of the invention relates to a variable compression ratio engine comprising a connecting rod via which a crankshaft that rotates about a crank axis reciprocates a piston within a cylinder. The connecting rod comprises a first part and a second part that are relatively positionable to set an effective length of the connecting rod and hence a compression ratio for the cylinder. A first locking mechanism is resiliently biased to an unlocked condition unlocking the first part from the second part when the two parts are in a position that sets a given effective length for the connecting rod to allow the two parts to be relatively positioned to a different effective length from the given effective length but is operable to a locked condition for locking the first part to the second part. A second locking mechanism is resiliently biased to a locked condition locking the first part to the second part when the two parts are in an effective length position different from the given effective length position but is operable to an unlocked condition that allows the first and second parts to be relatively positioned from the effective length position different from the given effective. length position. A first passage delivers hydraulic fluid to the first locking mechanism to operate the first locking mechanism from its unlocked condition to its locked condition in consequence of flow of hydraulic fluid to the first locking mechanism when the two parts are in the given effective length position. A second passage delivers hydraulic fluid to the second locking mechanism to operate the second locking mechanism from its locked condition to its unlocked condition in consequence of flow of hydraulic fluid to the second locking mechanism when the two parts are in the effective length position different from the given effective length position.
Further aspects will be seen in various features of presently preferred embodiments of the invention that will be described in detail.